1. Field of the Invention
The present invention relates to a novel photosensitive material, and a volume type phase hologram member formed by use of the same.
2. Related Background Art
Holography is a technique for recording an optical image by irradiating an object with a coherent light such as a laser beam; projecting onto a sensitive layer the light reflected by or transmitted through the object and modulated in terms of amplitude and phase corresponding to the shape of the object to be recorded; and illuminating the resulting hologram to reproduce the recorded optical image of the object. Thus, for example, a three-dimensional optical image can be viewed on a flat sheet film.
With development of researches on such holography, at present time it has been made considerably clear what are required for the sensitive materials therefor. Known as the sensitive materials usable in holography are various materials including silver halide emulsions, photoresists, thermoplastics, dichromated gelatin, inorganic glass materials and ferroelectric crystals, and researches are further being made on the suitability of the materials to the holography.
In recording a reflection hologram, in particular, as a volume phase type hologram, substrates are employed for holding and fixing sensitive materials in consideration of the required positional fixing of the sensitive material as the indispensable factor for light exposure because of the lattice distance of as minute as 0.1-0.3 .mu.m and need for fixing the sensitive material layer in the subsequent development by solvent treatment. Thus, glass substrates have been used in holograms formed of a sensitive material composed of gelatin, polyvinyl carbazole or the like, because one of the two light beams used for the exposure to light in forming the hologram is required to be transmissive through the substrate.
In particular, because of a strong hygroscopicity of the gelatin in instances in which a hologram recording layer comprises gelatin, the glass substrates have been considered to have the best performance to protect the recording layer from the surrounding moisture.
On the other hand, in instances in which the recording layer comprises a material being highly inert to water, care need not be taken as in the case of the gelatin.
In addition, employment of the glass substrates cannot obviate the problems of insufficient strength against stress such as impact, a danger when broken, in addition to the large weight.
From these viewpoints, it has been attempted to use substrates other than glass when the sensitive material comprises polyvinyl carbazole.
For example, U.S. Pat. No. 4,172,724 discloses that polyester is used as a substrate, and Japanese Patent Laid-open Publication No. 57-128378 (1982) discloses that methyl methacrylate resins, styrene resins, polyester resins and diethylene glycol allyl carbonate resins are used as substrates. Japanese Patent Laid-open Publication No. 59-119376 (1984) also discloses that CR-39 [a trade name of diethylene glycol bis(allyl carbonate)] is used as a substrate.
Of these polymers, the crystalline polymer polyester resins have a superior solvent resistance as compared with non-crystalline polymers, but may be affected by crystal orientation when exposed to light, since the substrates are formed by processing such as biaxial elongation. Since the crystals thus oriented may cause laser beams to be polarized and may further cause the beams to be scattered by the degree of chain orientation, such resins are not suited to the preparation of high-performance holograms.
The styrene resins and methyl methacrylate resins, which are amorphous and non-crosslinked polymers, are not necessarily uniform in their structure, and hence the non-uniformity of density causes the laser beams to be scattered, bringing about hindrance of exposure to light, and have a low solvent resistance and impact strength, disadvantageously.
The diethylene glycol bis(allyl carbonate), though it is a non-crystalline crosslinked polymer, has a refractive index as low as 1.5, so that a hologram formed by using it may result in spurious transmission or reflection hologram (undesired fringes) in the hologram.
For example, in display system using a reflection hologram, the spurious reflection hologram creats ghost images and the spurious transmission hologram creats rainbow-like flare patterns. Moreover the transmittance of the whole hologram may be decreased because of the angle dependence possessed by the spurious hologram.
The performances of holograms may be similarly lowered also when the undesired reflection diffraction fringes are formed in transmission holograms.